ICU-20939 Fix problem w regexp \b boundaries & UTF-8 text

ICU-20939 Fix problem w regexp \b boundaries & UTF-8 text

In regular expressions, when testing for word boundaries with \b, the
boundaries were incorrect when in Unicode mode, meaning that an ICU word break
iterator is being used to find the boundaries, and the text being matched is
UTF-8 encoded.

The bug stemmed from a misunderstanding of how string indexes work with UText
and break iterators, leading to the inclusion of code to convert from UTF-8 to
UTF-16 indexing, when what was wanted was the original UTF-8 index everywhere.
Removing the indexing conversion fixes the problem.